1. Field of the Invention
The present invention relates to a liquid crystal display device in which an oxide semiconductor is used and a manufacturing method thereof.
2. Description of the Related Art
As typically seen in a liquid crystal display device, a thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but can be formed over a large glass substrate. In contrast, a thin film transistor manufactured using crystalline silicon has high field effect mobility, but due to a crystallization step such as laser annealing, such a transistor is not always suitable for being formed over a large glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is manufactured using an oxide semiconductor and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor for an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.
The field effect mobility of a thin film transistor that uses an oxide semiconductor for a channel formation region is higher than that of a thin film transistor that uses amorphous silicon. The oxide semiconductor film can be formed by a sputtering method or the like at a temperature of 300° C. or less. Its manufacturing process is easier than that of a thin film transistor that uses polycrystalline silicon.
An oxide semiconductor is a transparent semiconductor which transmits light in a visible wavelength range; accordingly, it is said that by using the oxide semiconductor for a pixel of a display device, a higher aperture ratio can be obtained.
Such an oxide semiconductor is expected to be used for forming a thin film transistor on a glass substrate, a plastic substrate, or the like, and to be applied to a display device.